A rare and spectacular head-on collision between two galaxies appears in this Hubble telescope picture of the Cartwheel Galaxy, located 500 million light-years from Earth in the constellation Sculptor.

The striking ring-like feature is a direct result of a smaller intruder galaxy - possibly one of two objects to the right of the ring - that careened through the core [close-up image at lower left] of the host galaxy. Like a rock tossed into a lake, the collision sent a ripple of energy into space, plowing gas and dust in front of it. Expanding at 200,000 mph, this cosmic tsunami leaves in its wake a firestorm of new star creation. Hubble resolves bright blue knots that are gigantic clusters of newborn stars [close-up image at upper left] and immense loops and bubbles blown into space by exploding stars (called supernovae) going off like a string of firecrackers.